[USRP-users] (Mightbe) an impedance mismatching problem

Matt Ettus matt at ettus.com
Wed Jul 29 19:12:29 EDT 2015

You didn't say which daughterboard you are using, but I assume it is LFRX.
That has a 50 ohm input impedance, and your device might not be capable of
driving it.  You also need to make sure everything shares the same ground.


On Tue, Jul 28, 2015 at 5:48 AM, Jeon via USRP-users <
usrp-users at lists.ettus.com> wrote:

> I am building a photo-detecting circuit, whose output will be fed into a
> USRP N210. (Also have USRP1, E100, B200... But, currently I am using N210.)
> Here's a circuit that I built:
> To connect with the USRP (drawn as switch in the circuit. It's just a
> logical representation whether circuit output is fed into the USRP or not)
> I use BNC- Alligator CLIP SP RG58 coaxial cable of 1 meter. Since the USRP
> has the SMA port, I also put the BNC JACK to SMA PLUG adapter.
> A distributor says that the cable has an impedance of 50 ohm. The pages
> below are in Korean. Just in case.
> - BNC coaxial: http://eleparts.co.kr/EPX33HV8
> - BNCJ-SMAP adapter: http://eleparts.co.kr/EPX33PCF
> Back to the circuit, I've tested the circuit with another LED driving
> circuit. Without connecting the USRP.
> Blue is a state of an LED and yellow is a brightness reading (output of
> the op amp).
> I think it's not that pretty, but it's not that bad... It's OK...
> And then after, I connect the USRP at the output of the op amp. And did
> the same test. And the result is:
> First, it can be easily found that the blue reading and the yellow reading
> are not having a same HIGH/LOW pattern.
> In addition, a vertical scale is reduced to nearly 20 %. (The first
> oscilloscope has a scale of 500 mV, and the second one ahs a scale of 100
> mV.)
> With a little knowledge about circuit and hardware, I guess this problem
> is mainly caused by impedance mismatch.
> In this situation, I am curious whether this phenomenon is an impedance
> mismatch or not.
> If it is, how can I solve the problem with an arbitrary LED driving
> frequency?
> (Currently, the LED is driven with a frequency of 500 Hz, but I am
> planning to increase the frequency up to 800 kHz and more.)
> If it has nothing to do with impedance mismatch, what causes this
> phenomenon and how could I solve it?
> I apologize in advance if you feel this question does not suit this USRP
> users mailing list, but does suit other places such as EE stackexchange.
> Regards,
> Jeon.
> _______________________________________________
> USRP-users mailing list
> USRP-users at lists.ettus.com
> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.ettus.com/pipermail/usrp-users_lists.ettus.com/attachments/20150729/d3bd4360/attachment-0002.html>

More information about the USRP-users mailing list